How can I achieve a "painted" effect in my scene in blender?
The filters in Photoshop are your best bet. Gimp or Krita which are both free might also have such effects, but looking at the Blender compositon nodes i couldn't find any solutions. One thing that can be done in Blender are the lines using Freestyle. This will have the edges of all models show up and there are many advanced features regarding to this.